In an uplink line of 3GPP LTE (3rd Generation Partnership Project Long-term Evolution; hereinafter, simply referred to as “LTE”), a periodic sounding reference signal (P-SRS) is used as a reference signal for measuring uplink reception quality (refer to NPL 1).
In order to transmit the P-SRS from a terminal to a base station, SRS resources (hereinafter, referred to as “common resources”) which are common to all terminals are set. A notification of these common resources is performed with the cell units. For example, if a notification indicating that the common resources are first, third and eighth subframes is performed using control information, all terminals in a cell stop transmission of data signals during a predetermined time period (specifically, a final symbol) of each of the first, third and eighth subframes, and use the time period as a transmission resource of a reference signal.
In addition, information regarding a resource which is practically allocated to each terminal in the common resources is notified by a higher-order layer than a physical layer with the terminal units. Thereby, the common resources are distributed to the respective terminals.
Specifically, a resource for transmitting a reference signal can be periodically allocated to the respective terminals in the time axis direction.
In addition, methods of transmitting the reference signal include wide band transmission and narrow band transmission. In the wide band transmission, the reference signal is transmitted using the overall bands set for a terminal, whereas, in the narrow band transmission, the reference signal is transmitted using a first partial band of a plurality of partial bands forming set bands at the first timing, and the reference signal is transmitted using a second partial band different from the first partial band at the second timing. In other words, in the narrow band transmission, frequency hopping is employed (refer to FIG. 1). Therefore, the reference signal is transmitted using only the narrow band at one timing, but the frequency hopping is repeatedly performed, and thereby the reference signal is transmitted using all the set bands. Thereby, a base station can measure reception quality in all the set bands.
In addition, the reference signal is scrambled using orthogonal sequences in each terminal and is transmitted. In other words, the reference signals which are transmitted from a plurality of terminals are multiplexed through time division, frequency division, and code division, and are transmitted.
In addition, information (that is, a parameter used to specify a resource) regarding a resource which is practically allocated to each terminal in the above-described common resources includes a leading subframe, a set band, a transmission bandwidth, a frame interval at which a reference signal is mapped, a transmission time, and the like. Each terminal is notified of this information by a higher-order layer than a physical layer.
Here, as described above, the higher-order layer than the physical layer notifies of the information regarding the resource which is practically allocated to each terminal in the common resource. Since a terminal which has received the notification and a base station which is a transmission source transmit and receive ACK/NACK and the like therebetween in the higher-order layer, there is a problem in that time until the terminal practically starts transmitting, a reference signal after receiving the notification, is lengthened.
In an uplink line of LTE-Advanced (hereinafter, referred to as “LTE-A”) which has further progressed from the LTE, an introduction of a DA-SRS (Dynamic Aperiodic SRS) has been examined. Transmission timing of the DA-SRS is controlled by trigger information (for example, information of 1 bit) which is transmitted from a base station to a terminal. The trigger information is transmitted using a control channel (that is, PDCCH) of a physical layer. In addition, when the trigger information is received, the terminal starts transmitting the DA-SRS from each transmission antenna thereof. In addition, as methods of transmitting the DA-SRS, single shot in which one DA-SRS is transmitted with respect to a single piece of trigger information and multi-shot in which a plurality of DA-SRS are transmitted with respect to a single piece of trigger information are examined. In a case of the multi-shot, a plurality of DA-SRS are transmitted using different subframes. Further, parameters are independently set for the DA-SRS and the P-SRS.
Here, as described above, the P-SRS is periodically transmitted, whereas the DA-SRS is transmitted based on a notification which is performed from a base station to a terminal in a physical layer when transmission data is generated in bursts. Therefore, for example, the DA-SRS is used to measure reception quality for allocating a resource to a data signal which is generated in bursts and has a relatively small amount of data, whereas the P-SRS is used to measure reception quality for allocating a resource to a data signal which is frequently generated.